Electric motors that rotate a shaft part around a rotation shaft include double shaft electric motors in which, in addition to the case of using one end side of the shaft part as an output shaft, the other end side of the shaft part can also be used as an output shaft. In the following explanations, one end side of the shaft part is also referred to as the load side of the shaft part and the other end side of the shaft part is also referred to as the counter-load side of the shaft part. In double shaft electric motors, an opening is formed in a portion of the casing that overlaps with the rotation shaft on the counter-load side so that the shaft part can be guided to the outside of the casing also from the counter-load side.
In double shaft electric motors, when only the load-side of the shaft part is used as the output shaft, the shaft part does not project from the opening of the casing on the counter-load side in some cases in view of safety. In such a case, in order to ensure the environmental resistance (waterproof property, antifouling property, and the like) and safety, it is necessary to cover the opening formed on the counter-load side with a cover or the like. For example, Patent Literature 1 and Patent Literature 2 disclose technologies for covering the opening formed on the counter-load side with a cover or a bolt.